The problems associated with blind spots while moving a vehicle in reverse have long been recognized. There have been repeated accidents associated with vehicles backing-up involving personal injury and damage to property. As vehicles are generally operated by one person, drivers will often back-up without adequate visibility to the rear. Even the presence of a passenger who can help in spotting obstacles or a person standing behind the vehicle to guide the driver does not wholly eliminate the blind spot to the driver.
Technical aids to improve vision to the rear by short range obstacle or target detection and distance measurement have, therefore, been developed in recent years and introduced into the market. These devices generally fall into three categories: (i) optical systems using image sensors, infrared light, video or laser devices, (ii) conductivity measurement systems, and (iii) ultrasound systems.
As to the optical systems, U.S. Pat. Nos. 5,214,408 to Asayama and 5,304,980 to Maekawa disclose target detection systems using light sensitive image sensors for optically sensing two image signals from an object, comparing the two signals and calculating the distance to the object based on the deviation between the images using triangulation. The systems utilize dual lenses mounted at the front of the vehicle to measure the distance from a leading vehicle by comparing the two images received by each of the two lenses. An infrared light generating means is also provided for projecting infrared light toward the object when atmospheric light is insufficient. Some of the problems associated with these image sensor systems include poor resolution at very close distances which makes it unsuitable for backing-up applications; poor performance in situations of impaired visibility such as inclement weather, dust, smog and especially fog; the susceptibility of the lenses to damage or ineffectiveness due to dirt, dust, snow or rain; and the relative expense of such systems.
Another type of optical system involves video systems which consist of a camera mounted on the upper rear portion of the vehicle, and a monitor on the instrument panel or above the dashboard inside the vehicle. Video systems came into use in several special applications such as buses, airport apron vehicles, and the like. These devices have many disadvantages such as poor night vision, their inability to gauge the distance between the vehicle and the obstacle, and severe safety problems associated with the driver being distracted and preoccupied trying to observe the video monitor and use the side view mirrors while moving the vehicle in reverse.
Yet another optical system is a distance measurement system using lasers. These systems also suffer from the disadvantage of poor performance under adverse weather conditions, and are also unable to measure dose distances such as those under three meters.
Conductivity measurement systems as that disclosed in British Patent No. UK 2,266,397 are based on changes in the capacitance in the environment and are generally effective for a very short range, on the order of approximately 30 cm. The conductive strip must be mounted across the full width of the rear of the vehicle, and adverse weather conditions such as rain and snow may change the conductive field which may result in false alarms.
Ultrasonic systems are cited by industry experts as the preferred technology including ISO Technical Report ISO/TR 12155 published by the International Organization for Standardization. Various ultrasonic systems have been proposed, and include those employing basic ultrasonic signal transmitting and receiving circuitry. The technology is suitable for all weather and visibility circumstances, and provides the optimal performance in short distance measurement and target detection due to the lower speed of sound. A signal is transmitted and the echo is deflected to the receiver providing an indication of presence and distance of a target based on the speed of sound. The disadvantages of these devices in general include an inability to measure at very short distances due to cross-talk between the transmitter and receiver, and poor area coverage and the inability to detect targets of irregular shapes and/or sizes. These limitations are inherent in commercially available ultrasonic devices, systems or transducers whose lobes do not provide coverage of the entire blind spot behind a vehicle, and which are susceptible to failure when the transmitted signals are deflected from oddly shaped obstacles or targets. These drawbacks limit the usefulness of conventional ultrasonic devices to parking aids where any detected objects are large, such as loading docks or neighboring vehicles.
An example of a conventional ultrasonic system is disclosed in U.S. Pat. No. 4,467,313 to Yoshino et al. which employs three obstacle detectors mounted along the rear of a vehicle in perpendicular relation to the bumper. Each obstacle detector comprises an ultrasonic transducer for transmitting and receiving an ultrasonic wave and an analog switch, and covers a set watching area. One of the drawbacks of this system is that some areas not directly in front of a detector remain blind areas. Prior art FIG. 9 is a schematic representation of the angular configuration of the watching areas in which primed reference numerals are used. It is a top view of a portion of vehicle 10' having three obstacle detectors 12', 14' and 16' mounted in parallel, and an obstacle 18'. Each detector has an angular coverage area, and it can be seen that some areas close to the detectors remain uncovered. Those uncovered areas are labeled 20' and 22' and are indicated with hatching. Widening the angle of coverage to address this problem results in a significant decrease in the signal strength thereby reducing the distance that can be covered.
Another problem with conventional systems such as that pictured in FIG. 9 is that the use of transducers in parallel relation to one another may miss certain angular hazards due to deflection of the transmitted signal away from any receivers. In FIG. 9 two signals 24a' and 26a' are transmitted from detector 12' toward obstacle 18' which has an angular shape or orientation. Due to the shape of obstacle 18', deflected signals 24b' and 26b' travel away from any receiving sensors of the detectors and obstacle 18' is not detected.